1. Technical Field
One or more embodiments of the present invention relate to wireless communications technology. Specifically, one or more embodiments of the present invention are used to improve the performance of multi-user multiple-input, multiple-output (MU-MIMO) and single-user MIMO (SU-MIMO) systems that use a bit sequence representing the pre-coding matrix in order to represent the channel state information (CSI) of a user.
2. Background Art
Multi-user MIMO, and certain forms of Single-User MIMO, require that the transmitting entity know something about the channel state information (CSI) that exists between the transmitting antennas (e.g. at a base station) and the receiving antennas at one or more receivers (e.g. one or more mobile terminals). Multi-user MIMO (MU-MIMO), in particular, is sensitive to the accuracy in the knowledge of such CSI. Specifically, if the CSI that is assumed at the transmitter, and is used in selecting the users to be served, assigning rates to such users, and forming the beamforming vectors which carry data streams to such users, is different from the CSI that is experienced at the time and frequencies used for transmission, then high-levels of interference may be experienced between the streams of different users. This “intra” transmission interference may severely limit the rates that can be supported by MU-MIMO.
Many factors come together in determining the “accuracy” of the CSI known at the transmitter and receiver. Some of these depend on the physics of the channel, e.g. how fast the channel changes in time and/or frequency. Some depend on limits imposed by a system design, e.g. the time-delay between estimating the CSI at the receivers, feeding knowledge of such CS back to the transmitter, and ultimately using at the transmitter for directing data transmissions. Another factor is the number of bits used by a receiving entity to describe the CSI to the transmitter via digital feedback. The present release of 3GPP's LTE design uses only 4 bits to describe a rank-1 channel.
Part of the reason for using this potentially interference-limited 4-bit LTE design is that LTE has in the past placed strong emphasis on scenarios where either the CS changed rapidly in time and/or frequency, or scenarios where the channel feedback had to describe a state used over a large bandwidth (e.g. 5 to 20 MHz). In such a case channel dynamics dominated over CS feedback accuracy and MU-MIMO does not operate efficiently for such reasons rather than by feedback limitations. Another is a stronger focus on SU-MIMO which requires less accurate CS.
Even in cases with slower channel dynamics where MU-MIMO can operate efficiently, a difficulty in considering feedback in excess of 4 bits is designing large codebooks that also support rank n>1 feedback. For example, cases where a user may want to represent n=2 subspaces of the 4×2 CS. Other concerns include using a large number of CSI bits in terms of transmission resources and computational/receiver complexity.
However, such concerns are becoming less important as deployment scenarios look to lower channel dynamics such as lower mobility users and smaller cells. Furthermore, improvement in MIMO spectral efficiency for cases of multiple users ultimately rests on getting MU-MIMO to work efficiently over SU-MIMO improvements. Thus, emphasis is shifting to improving MU-MIMO, to allow for significant spectral efficiency over SU-MIMO. To do so, there is an inherent need for supporting more feedback bits in CSI.
LTE also presently supports a number of different codebook designs, including FFT and Householder transform-based designs. For operations with N transmit antennas, the designs define a number of orthonormal N×N “precoding matrices” (PMs), each selectable by a “Precoding Matrix Indicator” (PMI) which includes a bit sequence representing the selected code vector entry (precoding matrix) from the codebook. However, the LTE designs are rather constrained, where antenna elements are all given the same gain, and only small numbers of PMs are used. That is, there is no concept of a gain-vector across antennas. In addition, LTE and academic contributions have described designs using “intra-site” and “inter-site” antenna phase differences. However, such designs have not included a number of features described in this particular design and method.